Details

Description:

The title bar part of the decoration is missing [1]. It is not just transparent, as I can click through it.
The issue is present with some themes, e.g. present with elementary and not with zukitwo, sammy, or the default themes.
Additional info:

I don't think so. That sounds like xfwm4 died and thus all the window borders it renders were gone. In this bug, the window borders are only partially missing (and just for certain themes) and xfwm4 is not dead. (xfwm4 --replace doesn't help)